1. Field of the Invention
This invention relates to an apparatus for cooling molten plastic extrudate as it leaves the extruder in order to hold the size and shape of the extrudate as the molten plastic cools and hardens. The vacuum sizing apparatus has a vacuum tank containing water through which the extrudate passes as it cools and hardens. The vacuum tank has provisions to minimize the turbulence of the water within the tank and to prevent contamination of the water in handling small diameter tubing and other small size extrudate.
2. Description of the Prior Art
Efforts have been made in the plastics industry to provide for the controlled cooling and accurate sizing of molten plastic extrudate after it leaves the plastic extruder. The extrudate can easily be deformed if excessive pressure is applied to the outer surface of the extrudate. To prevent this type of deformation, vacuum cooling and sizing tanks have been developed that have a water bath within a vacuum chamber which prevents excessive pressure from being applied to the surface of the extrudate as it cools. An example of such an arrangement is found in U.S. Pat. No. 5,340,295 and entitled "VACUUM SIZING APPARATUS WITH CONTROLLED VACUUM".
When very small diameter tubing or other small extrudate is cooled in a vacuum chamber, it is essential to control the temperature of the water closely, and to minimize turbulence within the water. For some extrudate, it is desirable to provide for pre-skinning of the extrudate as it enters the vacuum chamber.
In prior cooling and sizing devices, unwanted turbulence occurs when water is added to the tank and when water is removed from the tank to control the water level. The present invention provides a vacuum tank for a vacuum sizing apparatus that is particularly suited for the cooling and sizing of small diameter medical tubing, electronic shrink tubing, and fiber optic cable where close tolerances are required for the finished product.